HV power supply build - circuit safety components.

Things at the limits.

Re: HV power supply build - circuit safety components.

Postby Doug Coulter » Sun Jun 26, 2011 5:19 pm

FWIW, we've found here that in a basic fusor type thing, that it takes around 16kv to see any neutrons at all. By 30, you can be sure even a fairly numb detector will count quick, and here the rate goes up much faster than proportional with increasing voltage. To get to either point with 10 ma or so takes some real fine gas control indeed, however. you can't get there with raw muscle (current) at too-high pressure, at least up to 50-60ma -- you just get more ions -- and the voltage might even drop some with increasing current limit.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: HV power supply build - circuit safety components.

Postby johnf » Tue Jun 28, 2011 1:40 pm

Dont know what happened to my last two replies but they aren't here


here goes again
Chris I use PKE1.5 400A's as high voltage zeners they can deal with a few mA continuous and handle 1.5kW peak during discharge

I use strings of these to protect the ion source ARC supplies that are @ terminal potential ie a 3kV 10mA supply floating above earth by terminal volts. During a terminal flashover it is possible to get say 85kV put on the output of the 3kV supply ---instant death but a chain of 8 of the afore mentioned diodes ensures that the ARC supply is happy.
We are learning that using isolating transformers providing isolated mains at terminal potential adds around 2000pf of charge storage and as the terminal volts go up so does damaging stored energy. Hence the use of motor alternator drives for terminal power
johnf
 
Posts: 433
Joined: Sun Aug 08, 2010 3:51 pm
Location: Wellington New Zealand

Re: HV power supply build - circuit safety components.

Postby chrismb » Fri Jul 01, 2011 11:31 am

Hello John, Can you link to these diodes spec-sheet and/or vendor? I have failed to find them with a search using the letter code.

In regards the 3kV supply sitting 'on-high', I recognise the simplicity of the 'Walton' mechanical solution to getting power to an HV potential, but I would also suggest that my capacitive-isolation solution might also provide a solid-state means to accomplish this. I reckon I could build a float-able 10mA 3kV supply with 120kV of capacitive isolation for about $10. Obviously, without a 90kV supply to test with, I'd not be able to confirm this. I recognise the physics of electrical fields around such potentials gets a bit strange, but it works to 30kV... I can't [yet] test higher than this...

I have been surprised at how small the coupling capacitances need be to accomplish the circuit, above. The supply ripple is dominated by the storage capacitor rather than the links, to a degree that I found surprising because one would tend to expect that the capacitor that 'lifts' the potential in the middle of the diode rectifier pair needs to have enough energy to keep the power flowing. But what actually happens is that as the cap 'looses' its voltage to the storage cap, so the ac supply driving it pushed to higher voltages which kinda drags the litte cap along with it. 30pF is enough to transmit 5mA at 40kHz into 10nF storage caps. Using 500pF has [in my tests] gains a mere 4% efficiency improvement over this, and using 10nF as the link cap gains only a further 2%, which are not efficiency levels I need concern myself with when 30kV 100pF caps are around 1/10th the price of 30kV 10nF!
chrismb
 
Posts: 620
Joined: Thu Aug 05, 2010 6:32 pm

Re: HV power supply build - circuit safety components.

Postby chrismb » Fri Jul 01, 2011 11:35 am

On the subject of these caps, I have some questions;

During my HV tests, I have seen how the diameter of the wires leading into the caps becomes the biggest issue for spark-overs, which I seem to be able to eliminate with judicious use of shrink tube in the right places. However, my plan is to use a series of caps to spread the voltage transitions and keep them below 10kV per cap.

My questions are; are there any other recommendations you might make to me to avoid arc-overs across components [capacitors] with high potentials across them, and is this the principal benefit of 'door-knob' capacitors or are there other benefits/deficits for door-knob caps compared with disc types?
chrismb
 
Posts: 620
Joined: Thu Aug 05, 2010 6:32 pm

Re: HV power supply build - circuit safety components.

Postby Doug Coulter » Fri Jul 01, 2011 2:08 pm

If you're only close to arc over, glyptal (the real thing, accept no substitutes) is a good insulator that can be dipped or brushed on -- a couple coats will hold off an extra few kv well. Maybe 10 if you do a nice job. It also cuts down sub-arc corona losses a lot. Back in the day, we always had some around the TV repair shop for this. It can stand pretty good heat to -- it thermosets (cross links) the first time it gets hot and becomes even better. Some of the two part potting compounds are good too, but not cheap at all and hard to work with. I got some stuff like Spellman uses and it's not easy to work with, as you have to put it under vacuum after mixing to get all the bubbles out, then you can pour it and pressurize it into the nooks and crannies. Regular silicone isn't so good. The hardware store stuff gives off ammonia to cure, the "GE type 2" is just a failed formula for most uses (including the intended one -- it's either hard when bought, or never gets hard). The only stuff I've had luck with is the blue Permatex gasket maker, which is pretty expensive -- and not as good as glyptal for either insulation or vapor pressure in vacuum uses.

The low losses on the pump caps is why people mostly are well satisfied with a plain CW multiplier -- they work, and you just move on to doing the experiments. The ripple on the main stack is because during one half cycle, the charge for the coupling caps is supplied by the filter caps below it....which is why most all supplies use split phase and as square a wave as they can -- then the ripple contributions of the two sides cancel. And you only need one big driver instead (much cheaper usually).

Somewhere above 30kv -- say 50kv and up for sure, the relationship between voltage and sparking length completely breaks down for all the but biggest, smoothest balls (and no dust or insects). What happens is "streamer formation" and once that begins, arc length is more or less unlimited. A little channel of ions forms (could be started by a cosmic ray) and it's conductive -- and has a very pointy end. The pointy end of course concentrates field around it, so makes more ions (drawing enough current to keep the channel ionized too) and the process can continue almost forever. I've had a 10 foot arc here on 100kv, was all too exciting -- it went out from the supply about 5 feet, made a u turn (right near a ground too!) and then came back about 5 more feet to hit about 6" from where it started. I've seen 300 foot arcs in movies on the 'net from a mere couple hundred kv when a breaker popped and the quench didn't work.

This is one of several reasons for corona rings on things like this -- to ensure that no single drop is large enough to cause this to happen. By dividing any big drop into definite, controlled, smaller drops, it's all much more manageable.

It's also how the voltage of mere lightning can make an arc to the ground....which is a good place to look for more information on the phenomenon.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: HV power supply build - circuit safety components.

Postby chrismb » Fri Jul 01, 2011 3:16 pm

Thanks, Doug, yes I could try glyptal. I'll think on it. I've never seen it outside the US before, though!

(....Just had a look and I can find a single UK supplier on the 'net, at ~$50 a litre.)
chrismb
 
Posts: 620
Joined: Thu Aug 05, 2010 6:32 pm

Re: HV power supply build - circuit safety components.

Postby Doug Coulter » Fri Jul 01, 2011 5:06 pm

It's expensive now - about the same here from Caswell plating. Used to be the stuff was on every street corner, the Army used it extensively in their electronics. Now the only place I know of that has it is Caswell - they sell it for doing authentic restorations of old things. Some "HV corona dope" is glyptal, but some isn't and also isn't so great.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: HV power supply build - circuit safety components.

Postby johnf » Sun Jul 03, 2011 4:35 pm

Chris

1.5KE400CA-E3 RS# 638-7583
johnf
 
Posts: 433
Joined: Sun Aug 08, 2010 3:51 pm
Location: Wellington New Zealand

Previous

Return to High Voltage, High Power

Who is online

Users browsing this forum: No registered users and 2 guests

cron